OBJECTIVE: To evaluate the effects of rapid maxillary expansion (RME) on nasal patency and nasomaxillary dimensions in children and adolescents with mouthbreathing through 8 years of clinical follow-up. METHODS: RME was performed using a Hyrax orthodontic appliance in 28 mouth-breathers (6-13 years old). During follow-up, objective tests of nasal respiratory function were conducted, such as acoustic rhinometry, which provided the minimum cross-sectional areas of the nasal cavity, and active anterior computed rhinomanometry, which measured inspiratory nasal resistance. The tomographic widths of the coronal sections of the nose and maxilla were also measured. Fisher's exact test and the Mann-Whitney U test were used to compare categorical and numerical variables, respectively, in mouth-breathers with and without allergic rhinitis. Temporal evolution was assessed using generalized estimating equation models. Statistical significance was set at RESULTS: There was a reduction in inspiratory resistance after RME with a stable improvement in nasal patency during the 8-year follow-up period ( CONCLUSIONS: Our study showed that RME promoted and maintained the widening of the posterior maxillary structure in children and adolescents with mouth-breathing, with a decrease in inspiratory nasal resistance during the 8-year follow-up period. These findings highlight the importance of RME in mouth-breathers with maxillary atresia.